FIELD OF THE INVENTION
The invention relates to a high-power grid-compatible converter-controlled, voltage-injecting phase-shifting transformer. The transformer includes a converter having an input side and an output side, an energizer transformer connected to the input side of said converter, and a booster transformer having at least one phase connected to the output side of said converter. The converter has an excitation-side power converter with at least one phase intercoupled by a DC intermediate circuit to an output-side power converter with at least one phase. Each of the excitation-side power converter and the output-side power converter has turnoff power semiconductors.
Because of pronounced intermeshing of the high-voltage grids, it is necessary to be able to control the power flow and its direction at will or as required on selected transmission lines. As a rule, use is made for this purpose of so-called phase-shifting transformer arrangements. They generally are formed of a three-phase energizer transformer and a three-phase booster transformer. The input winding of the energizer transformer draws electric power from the grid. The output winding is usually provided with a step switch. The winding supplies the electric power to the input winding of the booster transformer, whose output winding is connected in series to the transmission line and through which the entire line current flows. As a result, there is produced in the booster transformer an additional voltage whose absolute magnitude depends on the position of the step switch. Its phase angle is determined by the vector groups of the energizer transformer and booster transformer. The voltage across the output winding of the booster transformer is termed a phase-shifted voltage.
The functional principle of a power-flow controller is now described. A controllable voltage source is inserted in series into a transmission line. The power flow through the transmission line is influenced by specifically varying the absolute value and the phase angle of the series voltage. The phase angle of the series voltage relative to the current flowing in the line produces the effective power and the reactive power, which the voltage source exchanges with a grid. The effective power can either be fed to large energy stores or be drawn. If it is drawn, it is usually drawn from the grid and fed to the grid again via a second voltage source having a different voltage and connected in parallel with the grid. The use of a parallel voltage source is advantageous, since in addition to the effective power required in conjunction with the series connection, it is also possible to exchange reactive power with the grid. The reactive power exchange of the parallel connection is independent of that of the series connection.
The power-flux controllers can be constructed using voltage inverters. The power-flux controllers usually include as main components: a first transformer in a quadrature-axis branch, also denoted as an energizer transformer; a first inverter in the quadrature-axis branch, also termed an excitation-side power converter; a capacitive energy store; a second inverter in a direct-axis branch, also denoted as an output-side power converter; and a second transformer in the direct-axis branch, also denoted as a booster transformer.
The first and second inverters invert the DC voltage of the capacitive energy store into a stepped AC voltage. The AC voltage is matched via the transformation ratios of the energizer and booster transformers and to the grid voltages provided. Such a circuit is denoted as a Unified Power Flow Controller (UPFC). The article "The Unified Power Flow Controller: A New Approach to Power Transmission Control", published in IEEE Transactions on Power Delivery, Vol. 10, No. 2, April 1995, pages 1085 to 1093 discloses a UPFC circuit. The circuit includes an excitation-side inverter which is connected in parallel with the grid via an energizer transformer, an output-side inverter which is connected in series with the grid via a booster transformer, and a capacitor via which the two inverters are connected to one another on their DC voltage sides. As FIG. 1 of this article shows, each of the two power converters is configured as a 6-pulse three-phase bridge circuit with six turn-off power semiconductors, in particular gate turn-off (GTO) thyristor, a freewheeling diode being connected in anti-parallel to each GTO thyristor. In the discussions, the circuit is to be denoted as a two-point circuit, because it is possible by driving the GTO thyristors for the positive or the negative terminal of the energy store to be connected to the terminal of one phase of the three-phase system.
The achievable power of the two-point circuit is determined by the voltage endurance and the turn-off current of the GTO thyristor used. This holds when the GTO thyristor is sufficiently cooled. It may be assumed that this is presently capable of being implemented.
Since the invention relates to a power-flow controller of high power, the known options which can be used to increase the power of the overall system are set forth below. According to the prior art, two approaches may be adopted to provide a solution in this case. One approach leads via increasing the power of an inverter, the other approach utilizes the interconnection of a plurality of inverters.
If the required power of the UPFC circuits is higher than achievable with a GTO thyristor, a direct series connection of the GTO thyristor can be used. The higher power is achieved in this case by a higher voltage across the capacitive energy store.
The direct series connection has numerous advantages. First, the power of the two-point circuit can be raised in fine steps. Second, redundancy can be achieved by inserting additional GTO thyristors into the series connection.
On the other hand, the direct series connection of GTO thyristors has some disadvantages. For instance, when driving the GTO thyristors, it has to be ensured that all GTO thyristors connected in series switch exactly simultaneously, so that erroneous voltage distributions are avoided between the series-connected GTO thyristors. The coarse stair-step shape of the AC voltage of the power converter is not improved by an increase in the power. Generally, these circuits therefore cannot be used without additional filters for reducing the distortions in the grid voltage. The known damping circuits for the GTO thyristors in a direct series connection have substantially higher losses than those which can be used for individual GTO thyristors. The voltage utilization of the GTO thyristors drops with increasing number of series connections. This leads to a disproportionate increase in the number of series connections by comparison with the increase in the capacitor voltage, and this entails higher procurement costs and specific losses and thus higher operating costs for the circuit. To date, no number of series connections in excess of six has become known, and therefore the maximum achievable power of an inverter unit must be regarded as limited.
A further approach to increasing the power of an inverter is the indirect series connection of two GTO thyristors in the three-point circuit. In this circuit, the terminal of the phase of the three-phase transformer can be connected to the positive, negative or center terminal of the energy store. An example for the use of such a circuit is described in the article "Statischer Umrichter Muldenstein", ("Muldenstein Static Converter"), published in the German journal "eb-Elektrische Bahnen", Volume 93 (1995), Issue 1/2, pages 43 to 48. FIG. 4 of the article shows a four-quadrant controller in which two three-point phase modules are used. The extension of the three-point circuit to form an n-point circuit is described in the article entitled "Advanced Static Compensation Using a Multi-Level GTO Thyristor Inverter", IEEE 94 SM 396-2 PWRD, pages 1 to 7. The article sets forth advantages of the n-point circuit. First, the voltage of the inverter can be increased without a direct series connection of GTO thyristors, thus circumventing the need to switch the GTO thyristors exactly simultaneously. Second, the time-offset connection of the individual voltage stages permits a better approximation of the stepped AC voltage to the sinusoidal shape and thus a smaller proportion of higher harmonics in the output current. As a result, the GTO thyristors used can be better utilized and the fundamental power of the inverter can be increased. Third, it is possible using the n-point circuit for the power to be increased without additional inductive components (transformers or three-phase balance coils). Inductive components are more expensive to procure and have higher losses than comparable capacitive components. Fourth, the n-point circuit can be connected to the grid via a standard transformer. Fifth, the stepped AC voltage can be approximated to the sinusoidal shape without departing from the fundamental frequency modulation of the GTO thyristor drive. Fundamental frequency modulation means that each GTO thyristor is turned on or off only once per period. A higher-frequency GTO thyristor drive (pulse width modulation) is limited in practice to frequencies of up to 250 Hz because of the turn-off times and hold-off intervals of the components. Although this does permit a reduction in the low harmonics (mode numbers 5 to 17), it leads to an increase in the next higher harmonics (from 19th). This can entail an additional outlay on filters. A further important disadvantage of a higher-frequency GTO thyristor drive are the switching losses in the inverter, which rise in proportion to the switching rate.
The concept of the n-point circuit also has disadvantages. First, different requisite inverter powers require different numbers of points and, consequently, different structural configurations of an n-point phase module. Second, starting from n&gt;3, a higher voltage endurance is required in part than in the case of the diodes which lead to the intermediate terminals of the energy store. Third, the individual n-1 capacitor voltages must be controlled to the same level. This is difficult to achieve from a practical viewpoint. Fourth, a complicated drive logic of the GTO thyristors is required in order to exclude forbidden circuit states. A forbidden circuit state is understood to be the case where more than n-1 GTO thyristors arranged in series are turned on simultaneously.
It may be assumed that these disadvantages have so far prevented the implementation of numbers of points of n&gt;3.
Inverter systems which achieve a higher total power can be produced by interconnecting a plurality of inverters. Apart from increasing the power, the aim in this case is also to reduce the voltage distortions and to achieve a sinusoidal current.
Various possibilities for interconnecting inverters are known from the article entitled "A Comparison of Different Circuit Configurations for an Advanced Static Var Compensator (ASVC)", published in "PESC'92 Record", 23rd Annual IEEE Power Electronics Specialists Conference Toledo, Spain, 1992, pages 521 to 529. Either a two-point or a three-point inverter is used as inverter in the article. The GTO thyristors are controlled by fundamental frequency modulation, which results in each GTO thyristor being turned on and off only once per period.
Various possibilities for interconnecting two power converters are represented in FIG. 8 on page 524 of the article. As shown by the comparison of these alternatives carried out in the article, the highest fundamental power can be achieved with the circuit variants n and o. In these variants, use is made of two three-point circuits each, which are connected in series on the three-phase side via the primary windings of the transformers. The two transformers are configured in different vector groups (Yy and Yd), the result being a 12-pulse grid perturbation. As a result of the series connection of the inverters on the three-phase side, the inverter current contains no harmonics of mode numbers 5, 7, 17, 19 etc. The GTO thyristors used can thereby be utilized more effectively. The difference between the two variants n and o consists in the wiring of the DC voltage side, and this exerts no essential influence on the achievable power.
An SVG (Static Var Compensator) with a power of 80 MVA is known from the publication entitled "Development of a Large Static Var Generator Using Self-Commutated Inverters for Improving Power System Stability", printed in IEEE Transactions on Power Systems, Vol. 8, No. 1, February 1993, pages 371 to 377. In the case of this system, high power is achieved by using a series connection of six GTO thyristors per valve and a series connection of eight inverters via a special multi-winding transformer. As in the case of the variants n and o of the previously cited article, the increase in the fundamental power is achieved by turning the individual secondary windings of the transformer. The eight secondary windings of the transformer are turned relative to one another by 7.5.degree. el., and this leads to a 48-pulse grid perturbation.
In general, the advantages of increasing the power by series connections on the three-phase side are numerous. First, with the increase in the power, the grid distortions can be reduced by suitable transformer circuits. Second, the phase current of each individual inverter can be rendered more sinusoidal by the series connection, on the three-phase side, of the inverters via transformers, and the switching power of the GTO thyristor used can be utilized more efficiently thereby.
On the other hand, the circuit has disadvantages. First, the power can be stepped only coarsely by adding and omitting complete inverters. In the case of a direct series connection of GTO thyristors, a finer stepping can be performed by the number of GTO thyristors. Second, there is a need for highly specialized and therefore expensive transformers (special manufacture). Third, when a connection is made to the high-voltage grid, there is a need for an additional transformer in order to transform the supply voltage to an intermediate voltage for which the special transformer can be designed on the primary side. Fourth, the transformers are attended by relatively high operating losses.
German Utility Model Application G 94 16 048.1 discloses a static compensator (GTO-SVC) in which the power is increased by cascading power converter modules. A three-point phase module is used in this case as a power converter module, it is also possible to use two-point or, in general, n-point modules. A plurality of capacitors can be turned on or turned off in one phase one after another owing to the cascaded arrangement of the phase modules. The selection of the turn-on and turn-off instants can be made in such a way that the output voltage of the cascade inverter is approximated as well as possible to the sinusoidal shape. As a result, the proportion of higher harmonics in the current is reduced, and the utilization of the GTO thyristors is thus increased.
The advantages of this solution are numerous. First, the advantages of the n-point circuit come fully to bear. Second, the power is increased by multiple use of the same inverter module and the special development of different modules for different numbers of points is eliminated. Third, the individual capacitor voltages can be controlled independently of one another. Fourth, each inverter module represents a self-sufficient unit. No additional, prohibited circuit states can occur.
On the other hand, there are disadvantages. First, the separation of the three phases in conjunction with the cascading produces a higher outlay on capacitors. Second, the power can be stepped by adding or omitting individual cascades. Finer stepping can be achieved by the number of GTO thyristors in the case of direct series connection of GTO thyristors.